1. Field of the Invention
The present invention relates to a method for producing a molded article of a fiber-reinforced thermoplastic resin which is used as industrial materials including automobile parts such in exterior panels, structural parts and battery trays, and building materials such as access floors. More particularly, the present invention relates to a method for producing a molded article of a fiber-reinforced thermoplastic resin which is much less deformed due to orientation of the fibers and good in exterior gloss, dimensional stability and mechanical properties.
2. Description of the Related Art
Hitherto, some methods for producing a fiber-reinforced thermoplastic resin article have been proposed and industrially employed. One of the typical methods comprises molding the fiber-reinforced thermoplastic resin article from resin pellets which are reinforced with short-length fibers in a conventional molding manner such as injection molding. Another method uses thermoplastic resin pellets which are reinforced with middle-length fibers having substantially the same length as the pellets and produces the fiber-reinforced molded article by, for example, injection molding.
However, each of the above techniques has its own technical and economical problems.
In the method using the resin pellet reinforced with the short-length fibers which is most commonly employed, improvement of the mechanical strength which is the most important object of the fiber reinforcing, particularly impact resistance is not sufficient, although moldability, designs flexibility and the production cost of this technique are superior to other techniques. The reason for the insufficient increase of the mechanical strength is that the fibers are severely broken during two plasticizing and kneading steps, namely the step for mixing and dispersing the fibers and the resins (the granulating step), and the molding step. In addition, since the fibers flow in the mold together with the resin melt in the molding step, the orientation of the fibers remains in the molded article and causes large deformation of the molded article. When the fibers, in particular, the inorganic fibers are used, they will wear screws and cylinders of extruders and an injection molding machine which are used for granulation and molding. Such wearing of the apparatus is a big problem in view of the production cost of the molded article.
The method using the resin pellets reinforced with the middle-length fibers requires a specially designed extruder head, and the productivity is inferior to the method using the pellets reinforced with the short-length fibers. Therefore, the production cost increases. In addition, as in the case of the short-length fibers, the molded article tends to deform, and the screws and cylinders of the extruder and the injection molding machine are worn.
In addition to the above drawbacks, the molded article of the fiber-reinforced thermoplastic resin has a poorer appearance and gloss than a molded article of non-reinforced thermoplastic resin, since the fibers are exposed on the surface in the former.